Comprehensive Income (Reclassification Out of Accumulated Other Comprehensive Income) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Provision for income taxes	$ (19,198,798)	$ (40,601,708)	$ (59,658,782)	$ (117,317,687)
Amounts Reclassified from Other Comprehensive Income
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Total net reclassifications for the period	307,552	(269,971)	868,256	(175,306)
Amounts Reclassified from Other Comprehensive Income | Unrealized gains (losses) on available-for-sale debt securities
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Realized gain (loss) on sale of securities and interest rate swap	260,042	(176,777)	975,917	915,414
Provision for income taxes	(54,609)	61,872	(204,943)	(320,395)
Total net reclassifications for the period	205,433	(114,905)	770,974	595,019
Amounts Reclassified from Other Comprehensive Income | Unrealized gains (losses) on derivatives
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Realized gain (loss) on sale of securities and interest rate swap	129,265	(238,564)	123,142	(1,185,116)
Provision for income taxes	(27,146)	83,498	(25,860)	414,791
Total net reclassifications for the period	$ 102,119	$ (155,066)	$ 97,282	$ (770,325)